The present invention relates to a method and a device for stabilizing a motor vehicle.
German Published Patent Application No. 19 02 944 describes a device for preventing vehicles from skidding. The device contains measurement instruments for determining the instantaneous vehicle condition, connected to a control unit that responds to certain limit values of the vehicle. The device also includes means that can be switched on for automatic control of at least one device for keeping the vehicle on track, triggered by the control unit on reaching a predetermined limit value for the transverse acceleration. If a maximum possible transverse acceleration for a vehicle design is detected, the program for the control unit is set at a lower value. This means that the brakes are activated regardless of the driver""s reaction even below the critical threshold value, i.e., before the vehicle is oversteered, and the power regulating element of the machine is reset at a lower driving power.
German Published Patent Application No. 35 45 715 describes a device for regulating the drive of motor vehicles in the sense of maintaining stable driving conditions. This device includes a computing unit for determining a setpoint and a tolerance range for the rpm difference of the front wheels or the transverse acceleration or the yaw rate and a comparison unit, when this setpoint or tolerance range is compared with the actual value measured. The difference between the actual value and the setpoint or tolerance range serves as a control signal for the brakes of the wheels and/or for a power controlling element of the vehicle""s engine.
With the devices described above, the brakes of the wheels and/or a power controlling element for the engine are driven as a function of a comparison between an actual value of a quantity describing the transverse dynamics of the vehicle and a respective limit value so that the vehicle is stabilized on the basis of the reduction in vehicle speed. The vehicle speed resulting from the interventions in the brakes and/or the engine is not preset here.
The object of the present invention is to improve upon existing devices and methods for stabilizing motor vehicles to the extent that on the basis of the vehicle speed a defined condition is established for the vehicle for the case when a quantity describing the transverse dynamics of the vehicle is greater than or equal to a characteristic value for the quantity describing the transverse dynamics of the vehicle.
The method according to the present invention for stabilizing a motor vehicle is used in particular to prevent a vehicle from rolling over about a vehicle axis oriented in the longitudinal direction of the vehicle. To do so, a quantity describing the transverse dynamics of the vehicle is determined and is compared with at least one characteristic value, in particular a threshold value, for the quantity describing the transverse dynamics of the vehicle. For the case when the quantity describing the transverse dynamics of the vehicle is greater than or equal to the characteristic value, braking measures at least are implemented on at least one wheel and/or engine measures and/or retarder measures are implemented. These braking measures and/or engine measures and/or retarder measures are advantageously implemented in such a way that the speed of the vehicle is reduced to a preselectable speed value or is kept at a preselectable speed value.
Due to the fact that the speed of the vehicle is reduced to or kept at a preselectable speed value by the braking measures and/or by the engine measures and/or by the retarder measures, a defined condition is established for the vehicle in situations that are critical from the standpoint of transverse dynamics. For example, this defined condition may correspond to turning a curve at a maximum possible curve speed. In this case, the preselectable speed value corresponds to the maximum possible curve speed.
The speed of the vehicle is referred to below as the vehicle speed. At this point, the phrase xe2x80x9ca vehicle axis oriented in the longitudinal direction of the vehiclexe2x80x9d should be explained. First, the vehicle axis about which there is a tendency of the vehicle to roll may be the actual longitudinal axis of the vehicle. Second, it may be a vehicle axis which is twisted by a certain angle with respect to the actual longitudinal axis of the vehicle. It does not matter here whether or not the twisted vehicle axis passes through the center of gravity of the vehicle. The case of the twisted vehicle axis should also allow an orientation of the vehicle axis at which the vehicle axis corresponds to either a diagonal axis of the vehicle or to an axis parallel to the diagonal axis.
The value of the quantity describing the transverse dynamics of the vehicle, which is allowed for the vehicle without the vehicle becoming unstable on reaching this value, is used to advantage for the characteristic value. The term unstable is understood here to refer to the onset of skidding or rolling of the vehicle.
The characteristic value is either a fixedly predetermined value or a value determined for the respective driving condition of the vehicle. The fixedly predetermined value is determined in advance by driving trials, for example, and the resulting vehicle performance, and is supported by simulations. It is assumed that at this characteristic value the vehicle performance is stable in corresponding operating states where this value is reached. Or the characteristic value is determined for the respective vehicle condition. In other words, the characteristic value is determined during driving operation of the vehicle on the basis of the quantities determined for this driving operation.
The characteristic value is advantageously determined at least as a function of a quantity describing the wheel load of at least one wheel. A quantity describing the contact force of the respective wheel is used to advantage as the quantity describing the wheel load of the minimum of one wheel. This quantity is normally available in traction control systems.
Two alternative methods are available for determining the characteristic value. First, the characteristic value is determined as a function of the wheel load of at least one inside wheel in turning a corner and the quantity describing the transverse dynamics of the vehicle. If, as mentioned previously, the contact force of the respective wheel is used as the quantity describing the wheel load, a linear relationship between the quantity describing the transverse dynamics of the vehicle and the contact force is approximated to determine the characteristic value. The characteristic value is then obtained by interpolation, i.e., the characteristic-value corresponds to the value of the quantity describing the transverse dynamics of the vehicle at which the contact force is zero.
This procedure makes use of the fact that the instability of a motor vehicle is manifested first in wheel performance in situations that are critical from the standpoint of transverse dynamics. In other words, this type of determination yields a prompt and accurate measure of the maximum allowed transverse dynamics of a vehicle in the relevant situation. In situations that are critical from the standpoint of transverse dynamics, an imminent instability is manifested first on the inside wheels of the vehicle in turning a corner, so the characteristic value is advantageously determined as a function of a quantity describing the wheel load of an inside wheel in turning a corner.
In the second alternative, a quantity describing the mass of the vehicle is determined as a function of the quantities describing the wheel loads. Then the characteristic value is read out of an engine characteristics map with the help of the quantity describing the mass of the vehicle. The individual values of the engine characteristics map can also be determined in advance by driving tests supported by simulations. Therefore, the vehicle mass is used as a parameter because the vehicle mass influences the height of the center of gravity of the vehicle, which in turn influences the roll-over behavior of the vehicle and thus the maximum allowed transverse acceleration in turning a corner.
The two procedures mentioned have the advantage that in any situation that is critical for the transverse dynamics (in particular turning a corner at a high speed) the maximum allowed value prevails for the quantity describing the transverse dynamics of the vehicle, and thus the vehicle can be stabilized optimally by braking measures and/or engine measures and/or retarder measures corresponding to the respective driving situation.
As indicated by the preceding discussion, the characteristic value has the function of a limit value.
As part of the targeted reduction in vehicle speed mentioned above, the predetermined speed value is a fixedly preselected value, which was determined in advance by a method similar to that used to determine the characteristic value for the quantity describing the transverse dynamics of the vehicle on the basis of driving tests and with the help of simulations. Or the preselectable speed value is determined like the characteristic value with the help of an engine characteristics map. Or the preselectable speed value is determined during operation of the vehicle at least as a function of the characteristic value and/or a quantity describing the yaw rate of the vehicle. The two latter procedures have the advantage that in any situation that is critical from the standpoint of transverse dynamics, the maximum allowed value for the vehicle speed prevails, and thus the vehicle can be stabilized optimally by braking measures and/or engine measures and/or retarder measures corresponding to the given driving situation. In addition, in this way a speed value representing the maximum allowed driving speed in the corresponding driving situation, which is critical from the standpoint of transverse dynamics, is determined. This yields the additional advantage that the vehicle is not braked to an unnecessary extent. The vehicle can be driven at the maximum possible speed, and traffic flow is largely maintained.
At this point, the facts should be summarized again. The reduction in vehicle speed is initiated by observation of the transverse dynamics of the vehicle. The speed of the vehicle is reduced to a value that is determined by the transverse dynamics of the vehicle. Either this value is determined while the vehicle is being driven or it may be a predetermined value. In either case, it may be based on driving tests and/or simulations conducted in advance.
The braking measures and/or engine measures and/or retarder measures are preferably continued as long as the preselectable speed value is less than a quantity describing the vehicle speed.
A quantity describing the transverse acceleration of the vehicle is used as the quantity describing the transverse dynamics of the vehicle. However, in the method according to the present invention, the quantity describing the transverse dynamics of the vehicle is not measured directly with the help of a corresponding sensor means. Instead, it is determined at least as a function of a quantity describing the vehicle speed. Furthermore, the quantity describing the transverse dynamics of the vehicle is determined as a function of a quantity describing the yaw rate of the vehicle, with the quantity describing the yaw rate of the vehicle in turn being determined at least as a function of the quantity describing the vehicle speed and a quantity describing the steering angle of the vehicle. In other words, ultimately the quantity describing the transverse acceleration of the vehicle is determined as a function of the vehicle speed and the steering angle.
This procedure in determining the quantity describing the transverse acceleration is associated with a definite time advantage with regard to supplying the signal for this quantity. This can be explained as follows. Normally, turning a corner is initiated by setting a steering angle. A corresponding transverse acceleration results from this cornering. If this resulting transverse acceleration is detected with the help of a transverse acceleration sensor, a considerable amount of time elapses between the setting of the steering angle and supplying of the transverse acceleration signal by the transverse acceleration sensor. This is due, inter alia, to the time sequence between the setting of the steering angle and the resulting reduction in the transverse acceleration and also due to the inertia of the transverse acceleration sensor. This time lag is largely eliminated by the procedure described above in determination of the quantity describing the transverse acceleration, in other words, immediately after the steering angle has been set, the prevailing value of the transverse acceleration is the value established in the resulting steady state of the vehicle on the basis of the steering angle set. Since the device and the method according to the present invention prevent roll-over of the vehicle about a vehicle axis oriented in the longitudinal direction of the vehicle, the procedure described above is possible in determining the quantity describing the transverse dynamics of the vehicle, i.e., the quantity describing the transverse acceleration, because roll-over of a vehicle usually occurs when turning a corner, and cornering is performed on the basis of a steering angle selected by the driver of the vehicle.
In summary, it can be concluded that the quantity describing the transverse dynamics of the vehicle and/or the quantity describing the yaw rate of the vehicle is advantageously determined with the help of simple mathematical models describing a steady state of the vehicle. This yields the time characteristic described above.
Due to the braking measures described above, all the wheels of the vehicle are advantageously braked uniformly. Uniform braking should be understood to mean that different braking forces are not intentionally set from the beginning. As an alternative to or in support of these braking measures, the moment delivered by the engine is reduced through appropriate engine measures. The vehicle speed is reduced or kept at a preselectable speed value as a result of these two measures. It is also possible to implement such braking measures where at least the rear inside wheel in turning is braked less than the other wheels of the vehicle and/or not at all. With these types of braking measures, a temporary increase in yaw rate during the braking measure and thus a resulting unstable state are prevented.
If the latter types of braking measures are selected, then advantageously all steps between a normal braking measure and no braking measure are possible on the rear inside wheel in turning. The step at which the rear inside wheel in turning is to be braked can be determined as a function of a quantity describing the transverse dynamics of the vehicle, for example.
As an alternative to the procedure described previously, the characteristic value is reduced by a small value. The quantity describing the transverse dynamics of the vehicle is compared with the reduced characteristic value. For the case when the quantity describing the transverse dynamics of the vehicle is greater than the reduced characteristic value, the speed of the vehicle is reduced to a preselectable speed value at least by braking measures on at least one wheel and/or by engine measures and/or by retarder measures. As a result of this procedure, the vehicle speed is reduced not when the characteristic value has been reached but at a slightly earlier point in time, namely when the vehicle approaches a driving condition which is critical from the standpoint of transverse dynamics as described by the characteristic value.